The use of turbochargers to increase power output and decrease fuel consumption in four-cycle internal combustion engines is common practice today. Both spark ignition and diesel engines use turbochargers to advantage and, in the case of diesel engines, the power output of an engine of a given cylinder displacement can easily be doubled by the addition of turbocharging with aftercooling. The turbocharger has gone through decades of development, and modem turbochargers used on high-speed diesel and gasoline engines are relatively low in cost and high in efficiency, and are durable commercial products.
Although the turbocharger utilizes exhaust gas energy that would otherwise be wasted, the imposition of an exhaust gas turbine in the engine exhaust system necessitates raising the average back pressure on the engine cylinders in order to generate sufficient pressure drop across the turbine to generate the power necessary to drive the turbocharger's compressor. This back pressure acts against the upstroke of the piston as it forces residual products of combustion out of the cylinder through the exhaust valves and increases the pumping loss of the engine. The level of back pressure caused by high pressure turbocharging of four-cycle engines is very high, even with the use of turbochargers that have relatively high overall efficiency. Any means that may be employed to lower the back pressure caused by the turbocharger turbine can result in significant improvement in engine performance. For example, if a diesel engine requires a pressure ratio of 2.5 times atmospheric pressure to reach the desired rated engine power output, a single turbocharger would impose a back pressure in the exhaust system of approximately two times atmospheric pressure.
The use of series turbochargers is common today on engines that are rated in high power output. If the two compressors are placed in series combination, the pressure ratios of the compressors are multiplied so high supercharge pressure can be supplied to the engine beyond that which a single turbocharger could produce by itself. If, for instance, a highly rated engine requires 4.5 pressure ratio, which is beyond the capability of a single commercial turbocharger, series turbochargers can provide a low pressure stage of 2.1 pressure ratio and a high pressure stage of 2.15 pressure ratio, the product of which is 4.51 pressure ratio overall. This, however, significantly raises the exhaust gas back pressure.